Mahiko Goto

Infarct size limitation by a new Na^+-H^+ exchange inhibitor, Hoe 642 : Difference from preconditioning in the role of protein kinase C

OBJECTIVES This study examined the effect of a new specific Na(+)-H+ exchange inhibitor, Hoe 642, on infarct size and the protective role of protein kinase C (PKC) by this agent. In addition, we assessed the possible alteration of Hoe 642-induced cardioprotection by commonly used animal anesthetic drugs. BACKGROUND Earlier studies on the contribution of Na(+)-H+ exchange to ischemic injury were complicated by nonspecific actions of the Na(+)-H+ exchange inhibitors, and the role of this exchanger in myocardial infarction in vivo remains unclear. The difference in anesthetic agents used in experiments could have resulted in discrepant findings regarding cardioprotection of some interventions, such as preconditioning and adenosine triphosphate-sensitive potassium channel openers. METHODS Infarction was induced by 30 min of coronary occlusion and 3 h of reperfusion in the rabbit heart. In the first series of experiments, rabbits were anesthetized with pentobarbital or ketamine/xylazine. Hoe 642 was injected intravenously 10 min before ischemia or 5 min before reperfusion. In the second series of experiments, rabbits received 25 mg/kg body weight of polymyxin B (polyB), Hoe 642 plus polyB, preconditioning with 5 min of ischemia and 5 min of reperfusion plus polyB or preconditioning alone before 30 min of ischemia. RESULTS In pentobarbital-anesthetized rabbits, 0.3 mg/kg and 0.6 mg/kg of Hoe 642 given before ischemia limited infarct size (as percent area at risk [%IS/AR]) to 42.7 +/- 4.4% (SEM) and 26.2 +/- 5.4%, respectively, from the control value of 55.1 +/- 3.5%. However, injection of Hoe 642 before reperfusion did not change infarct size (%IS/AR 49.6 +/- 4.9%, p = 0.387; power 0.81 for detecting 50% reduction). Infarct size limitation by the preischemic treatment with Hoe 642 was similarly observed in the rabbits anesthetized with ketamine/xylazine. In the polyB-treated rabbits, 0.6 mg/kg of Hoe 642 significantly limited infarct size (%IS/AR was 28.3 +/- 3.8% with Hoe 642 and 50.1 +/- 7.5% without Hoe 642), although preconditioning was blocked by the same dose of polyB (%IS/AR was 39.3 +/- 6.1% with polyB and 11.3 +/- 2.4% without polyB). CONCLUSIONS Hoe 642 enhanced myocardial tolerance against infarction, and this enhanced tolerance was not influenced by anesthetic agents commonly used for infarct size studies. Infarct size limitation by Hoe 642 was not inhibited by polyB, suggesting that cardioprotection by Na(+)-H+ exchange inhibition is not PKC mediated and thus may be unrelated to preconditioning.

Mitochondrial ATP-sensitive K+ channels play a role in cardioprotection by Na+-H+ exchange inhibition against ischemia/reperfusion injury.

OBJECTIVES The possible role of the ATP-sensitive potassium (KATP) channel in cardioprotection by Na+-H+ exchange (NHE) inhibition was examined. BACKGROUND The KATP channel is suggested to be involved not only in ischemic preconditioning but also in some pharmacological cardioprotection. METHODS Infarction was induced by 30-min coronary occlusion in rabbit hearts in situ or by 30-min global ischemia in isolated hearts. Myocardial stunning was induced by five episodes of 5-min ischemia/5-min reperfusion in situ. In these models, the effects of NHE inhibitors (cariporide and ethylisopropyl-amiloride [EIPA]) and the changes caused by KATP channel blockers were assessed. In another series of experiments, the effects of EIPA on mitochondrial KATP (mito-KATP) and sarcolemmal KATP (sarc-KATP) channels were examined in isolated cardiomyocvtes. RESULTS Cariporide (0.6 mg/kg) reduced infarct size in situ by 40%, and this effect was abolished by glibenclamide (0.3 mg/kg), a nonselective KATP channel blocker. In vitro, 1 microM cariporide limited infarct size by 90%, and this effect was blocked by 5-hydroxydecanoate (5-HD), a mito-KATP channel blocker but not by HMR1098, a sarc-KATP channel blocker. Infarct size limitation by 1 microM EIPA was also prevented by 5-HD. Cariporide attenuated regional contractile dysfunction by stunning, and this protection was abolished by glibenclamide and 5-HD. Ethylisopropyl amiloride neither activated the mito-KATP channel nor enhanced activation of this channel by diazoxide, a KATP channel opener. CONCLUSIONS Opening of the mito-KATP channel contributes to cardioprotection by NHE inhibition, though the interaction between NHE and this KATP channel remains unclear.

Limitation of myocardial infarct size by adenosine A1 receptor activation is abolished by protein kinase C inhibitors in the rabbit

OBJECTIVE The aim was to test whether infarct size limitation by adenosine A1 receptor activation is mediated by protein kinase C. METHOD In the first series of experiments, myocardial infarction was induced in rabbits under pentobarbitone anaesthesia by 30 min coronary artery occlusion and 3 h reperfusion. Rabbits were pretreated with no drugs (control). 1 mg.kg-1 of R(-)N6-2-phenylisopropyl adenosine (PIA), 50 micrograms.kg-1 of staurosporine (Stauro), 2.5 mg.kg-1 of polymyxin B (PolyB), and a combination of PIA and either Stauro or PolyB. Infarct size and the area at risk were determined by tetrazolium staining and fluorescent particles, respectively. In the second series of experiments, the inotropic response to 4 beta-phorbol 12-myristate 13-acetate (PMA) was assessed in rabbits with and without pretreatment using the same doses of Stauro and PolyB as those in the first series of experiments. RESULTS Infarct size expressed as percent of area at risk (%IS/AR) was significantly smaller in the PIA treated group than in the control: %IS/AR = 19.0(SEM 2.4)% v 37.7(4.0)%, P < 0.05. However, %IS/AR in the groups given Stauro [35.0(4.2)%], PolyB [37.9(3.2)%], PIA plus Stauro [34.9(3.9)%], and PIA plus PolyB [36.3(3.3)%] did not differ from the control value. PMA at the dose of 0.02 and 0.05 micrograms.kg-1 caused a dose dependent increase of the left ventricular dP/dtmax in the untreated rabbits. Such a positive inotropic response to PMA was not detected in rabbits pretreated with Stauro or PolyB, suggesting that the doses of the protein kinase C inhibitors were appropriate to block protein kinase C in the heart. CONCLUSIONS Infarct size limitation by A1 receptor stimulation is mediated by activation of protein kinase C in pentobarbitone anaesthetised rabbits.

Effect of protein kinase C inhibitors on cardioprotection by ischemic preconditioning depends on the number of preconditioning episodes

OBJECTIVES This study examined the possibility that the role of PKC in [corrected] PC, and thus the response to PFC inhibitors, may differ depending on how many ischemic episodes are employed to precondition the heart. METHODS In the first series of experiments, myocardial infarct was induced by 30 min of coronary occlusion and 3 h of reperfusion in the rabbit. Infarct size was determined by tetrazolium staining and expressed as a percentage of area at risk (%IS/AR). Prior to the 30-min ischemia, rabbits were subjected to no PC, single PC (i.e., PC with an episode of 5 min ischemia/5 min reperfusion), and repetitive PC (2 cycles of 5 min ischemia/5 min reperfusion) with or without one of three treatments: polymyxin B (PolyB), staurosporine (Stauro), and 8-sulfophenylthephylline (SPT). In the second series of experiments, the rabbits received 5 min of coronary occlusion after repetitive PC with or without PolyB or Stauro treatment. Then, myocardial tissue in the ischemic region was sampled for assay of PKC activity. Untreated rabbits served as controls. RESULTS Single and repetitive PC limited %IS/AR to the same extent (%IS/AR = 9.8 +/- 1.9 and 10.4 +/- 2.3, both p < 0.05, vs. the control value of 44.5 +/- 3.4), and single PC was blocked by PolyB (%IS/AR = 43.9 +/- 2.7) and Stauro (%IS/AR = 31.5 +/- 3.2). Although the protocol of PolyB injection maintained the plasma PolyB level during sustained ischemia well above its Ki for PKC, this agent and also Stauro failed to abolish the protection by repetitive PC (%IS/AR = 21.6 +/- 3.0 and 11.4 +/- 4.3, respectively). SPT, an adenosine receptor antagonist, not only blocked single PC (%IS/AR = 44.4 +/- 4.4) but also attenuated protection by repetitive PC (%IS/AR = 28.3 +/- 3.6). Infarct sizes in non-preconditioned hearts were not modified by PolyB, Stauro, or SPT. The ratio of membrane fraction PKC activity to cytosolic fraction PKC activity was elevated by repetitive PC plus 5 min ischemia, and this change in PKC was inhibited in hearts given PolyB and Stauro. CONCLUSIONS In contrast to single PC, repetitive PC protects the heart against infarction even when PolyB and Stauro are administered to inhibit PKC during ischemic insult. This difference may be attributable to a PKC-independent mechanism, in which the adenosine receptor may be partly involved.

Glibenclamide, a blocker of ATP-sensitive potassium channels, abolishes infarct size limitation by preconditioning in rabbits anesthetized with xylazine/pentobarbital but not with pentobarbital alone.

The role of ATP-sensitive potassium channels (KATP) in the mechanism of ischemic preconditioning (PC) is controversial, partly because descriptions of inhibition of PC by KATP blockers in the literature are inconsistent. We sought a reason for the discrepant findings regarding the effects of glibenclamide (Glib), a specific blocker of KATP, in preventing the reduction of infarct size (IS) induced by PC. The effect of Glib pretreatment (0.3 mg/kg i.v.) on PC was examined in three conditions: (a) when PC was performed with 3- and 5-min ischemia (i.e., potency of PC differs), (b) when rabbits were pretreated with prazosin and metoprolol (0.15 mg/kg i.v. each) to reduce myocardial O2 consumption, and (c) when xylazine was added to pentobarbital anesthesia. In rabbits under pentobarbital anesthesia, the left coronary artery was occluded for 30 min and then reperfused. The area at risk (AAR) and IS were determined 72 h after reperfusion in the first series of experiments and 3 h after reperfusion in the second and third series. IS as a percentage of AAR (%IS/AR) were 31.7 +/- 2.8 and 19.6 +/- 2.5% (SEM) after PC with 3- and 5-min ischemia, respectively, values significantly smaller than %IS/AR in the untreated control group (49.2 +/- 3.3%). The limitation of IS observed with 3- or 5-min PC was not prevented by Glib. Glib also failed to block %IS/AR reduction by PC, even when rate-pressure product (RPP) was reduced to approximately 65% by prazosin/metoprolol (Praz/Met) pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of regional contractile function by preconditioning ischemia does not play a permissive role in the infarct size-limitation by the preconditioning

SummaryAlthough previous studies have shown that preconditioning cannot be explained by concurrent myocardial stunning alone, it remains unclear whether reduction of contractile function by preconditioning ischemia is required for its cardioprotective effect. The present study examined whether preconditioning occurs in the absence of regional contractile dysfunction.In the first series of experiments, rabbits received two cycles of 2-min coronary occlusion separated by 5-min reperfusion, with or without dobutamine infusion (10 μg/kg/min, i.v.) commencing before the onset of ischemia. Regional thickening fraction measured by epicardial Doppler sensor was 72.8±4.7% of baseline (mean±SEM) in the untreated group and 102.9±3.1% in the dobutamine group at the end of the second cycle of ischemia/reperfusion. In the second series of the study, four groups of rabbits underwent 30-min coronary occlusion and reperfusion. The control group was untreated, and the PC group was preconditioned with two cycles of 2-min ischemia/5-min reperfusion before the 30-min ischemia. The PC-DOB group received both preconditioning and dobutamine infusion (10 μg/kg/min, i.v.), which was started 5 min before the preconditioning and continued for 19 min. The DOB group was given dobutamine infusion like the PC-DOB group, but was not preconditioned. After 72-h reperfusion, infarct size and area at risk were determined by histology and fluorescent particles, respectively. Infarct sizes in the PC and PC-DOB groups (25.0±3.4% and 22.7±3.3% of area at risk, respectively) were significantly smaller than that in the control group (48.2±2.6%). In the DOB groups, infarct size (43.5±4.0%) was similar to the control value.Infusion of dobutamine at a dose sufficient to abolish the contractile dysfunction which would have been induced by ischemic preconditioning did not attenuate the infarct size-limiting effect of preconditioning. Thus, it is unlikely that reduction of contractile function plays a permissive role in the appearance of the cardioprotective effect of preconditioning.

Does verapamil limit myocardial infarct size in a heart deficient in xanthine oxidase

1. The delay of ischaemic myocardial necrosis by verapamil has been reported in the dog heart, which contains a high level of xanthine oxidase, a potential source of cytotoxic free radicals. To test whether the retardation of ischaemic myocyte death by verapamil is not an isolated phenomenon in the xanthine oxidase rich heart, we assessed the effect of verapamil in the rabbit heart, which lacks xanthine oxidase.